Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. As design rules and process windows continue to shrink in size, inspection systems are required to capture a wider range of physical defects while maintaining high throughput. In addition, memory and logic architectures are transitioning from two dimensional floating-gate architectures to fully three dimensional geometries. In some examples, film stacks and etched structures are very deep (e.g., three micrometers in depth, and more). Measurement of defects buried within these structures is critical to achieve desired performance levels and device yield, yet these measurement have proven challenging for traditional measurement systems and techniques.
In some examples, electronic tests are employed to detect buried defects. However, multiple device layers must be fabricated before electronic tests are performed. Thus, defects cannot be detected early in the production cycle. As a result, electronic tests are prohibitively expensive to perform, particularly during research and development and ramp phases of the production process, where rapid assessment of defects is critical.
In some other examples, wafers are de-processed to uncover buried defects. Wafer de-processing destroys the wafer by removing layers to reveal defects-of-interest (DOI) detected using traditional optical or electron beam inspection. This approach is very slow, requires alternate process flows at each layer, and the alternate processes may produce defects that interfere with DOI detection. In addition, some DOI on some layers are not easily revealed by wafer de-processing.
In some other examples, buried defects can be detected based on x-ray based measurement techniques. For example, an x-ray diffractive measurement system or a coherent x-ray imaging system may be employed to detect buried defects. X-ray based measurement techniques have the advantage of being non-destructive, but throughput remains quite low.
In some other examples, electron beam inspection (EBI) is employed directly to detect buried defects. However, EBI is extremely limited in its ability to detect defects beyond a depth of approximately one micrometer. In many examples, EBI is limited to depths that are far less than one micrometer (e.g., less than fifty nanometers). This limitation is due to practical limits on electron dosage before sample distortion or destruction occurs. Thus, EBI is limited in its effectiveness as a defect detection tool for thick, three dimensional structures.
Some traditional optical inspection techniques have proven effective for the detection of defects buried in relatively thick layers. In one example, confocal optical inspection is employed at different depths of focus. Confocal imaging eliminates spurious or nuisance optical signals from structures above and below the focal plane. The confocal optical inspection technique is described in further detail in U.S. Patent Publication No. 2014/0300890, which is incorporated herein by reference in its entirety. In another example, a rotating illumination beam is employed to detect buried defects in relatively thick layers. Optical inspection utilizing a rotating illumination beam is described in further detail in U.S. Patent Publication No. 2014/0268117, which is incorporated herein by reference in its entirety. In another example, different illumination wavelength ranges are employed to detect buried defects as described in further detail in U.S. Pat. No. 9,075,027, which is incorporated herein by reference it its entirety. In yet another example, multiple discrete spectral bands are employed to detect buried defects as described in further detail in U.S. Pat. No. 8,912,495, which is incorporated herein by reference it its entirety.
Although traditional optical inspection techniques have proven useful for detecting possible defects in thick layers, the measurement results are typically insufficient to identify the defect as a defect of interest and classify the defect with a high degree of confidence.
In some examples, the optical measurement results are accepted without verification. However, making process decisions based on unverified optical measurement results runs the risk of introducing process errors that lead to lost time and resources.
In some examples, an optical inspection tool records the location of defects detected on a wafer. The wafer is subsequently transferred to a focused ion beam (FIB) machining tool, along with the recorded locations. The FIB tool machines away layers of wafer material to reveal the potential defects-of-interest (DOI). The potential DOIs are subsequently inspected by traditional optical or electron beam inspection techniques (e.g., scanning electron microscopy).
Unfortunately, the rate of material removal of a FIB tool is very low. In addition, the FIB tool is limited in its ability to locate the optically detected defects with an accuracy of approximately one micrometer. Due to this uncertainty, a significant amount of time is required to machine away material before the actual defect location is identified. Typically, FIB processing of one defect requires approximately one hour, if the defect can be found at all.
Improvements in the detection of defects of interest buried in vertical semiconductor devices, such as 3D memory, VNAND memory, or other vertical structures, are desired.